In the sixties of the last century, a debate was going on with respect to the use of the plastic method of structural design. It was stated that the upper and lower bound methods, neglected the “real behaviour of the structure”. In Delft the professor in Structural Mechanics at that time, Prof. Vreedenburgh, counteracted this criticism with “The structure just laughs about the way we calculate it”.
I had to think of this famous 60-year-old phrase again during recent discussions on the use of the Partial Factor Method in combination with Non-linear Finite Element models. It was decided to write an informative Annex on this topic as part of the second edition of Eurocode EN1990, Basis of Structural and Geotechnical Design. For a long time, papers and reports have been published in this area, but it still seems difficult to formulate “common unified rules” for all materials and structures. It turned out that serious discussions and deliberations were necessary for reaching a minimal degree of agreement about the requirements as well as the numbers. During the debates in particular the following sentence drew my attention: Maybe we should first calculate the “real behaviour of the structure” and then adopt margins to account for possible uncertainty issues. Formulated this way, it seems that reality and the deviations thereof belong to different worlds. I would prefer the statement that uncertainties are an essential part of reality. In order to arrive at responsible decisions for design and assessment, an integrated and preferably quantitative treatment of all uncertainties is mandatory.
Of course, when preparing the new edition of EN 1990 many other issues came up. Some were resolved within the redrafting process, but not all and certainly not all completely. There is, so to speak, still room for improvement. Typical examples for further development are the assessment of existing structures, the uncertainties related to prestressing, the combination of degradation and extreme loading, the choice of the basic reference period for the failure probability (one year or 50 year), modelling of imposed loads in buildings and robustness. Hopefully, we can use the present momentum to continue the research on these issues, among others also within IABSE.
At the same time, the codification world faces a set of new challenges. New paradigms like resilience and sustainability have popped up, setting new objectives for the design, asking for new models and (not to forget!) ways to deal with related new uncertainties. Again the joint agreement, necessary to produce a code, may turn out to be difficult. It is good news that the Liaison Committee of the professional associations IABSE, CIB, fib, ECCS, RILEM and IASS has decided to start up (in the same way as the Joint Committee on Structural Safety 50 years ago) the Joint Committee on GLOBE Consensus (GLOBE for short) to join forces on sustainability. I hope that this cooperation will result in useful recommendations for the engineering profession and for society in general.
Ton Vrouwenvelder, The Netherlands
Recipient of the IABSE International Award of Merit 2023
Ton Vrouwenvelder